Fas (also known as CD95 or APO-1) is a widely expressed cell death receptor that has a critical role in the regulation of the immune system and tissue homeostasis. Fas is activated by Fas ligand (FasL), a trimeric transmembrane protein (reviewed by Nagata, Cell 88:355-365, 1997). Fas is thought to have an essential role in deleting autoreactive lymphocytes and maintaining peripheral tolerance. Inherited Fas mutations in humans and mice cause a syndrome of massive lymphoproliferation and autoantibody production (reviewed by Nagata, 1997). Fas-induced apoptosis is also a major mechanism in cytotoxic T lymphocyte-mediated cytolysis and in the maintenance of immune privilege sites (reviewed by Abbas, Cell 84:655-658, 1996). Moreover, depending on the signal from the B cell antigen receptor, Fas may induce either apoptosis or proliferation of B cells in vivo (Rathmell et al., Cell 87:319-329, 1996).
Fas belongs to the tumor necrosis factor (TNF) receptor superfamily, which includes TNF receptor 1 (TNFR1), TNFR2, CD40, and the p75 low affinity NGF receptor; these receptors share characteristic cysteine-rich repeats in their extracellular domains (reviewed by Smith et al., Cell 76:959-962, 1994). The intracellular tails of Fas and TNFR1 share homologous death domains, an approximately eighty amino acid protein motif that is critical for signaling apoptosis (Itoh and Nagata, J. Biol. Chem. 268:10932-10937, 1993; Tartaglia et al., Cell 74:845-853, 1993). Over the last two years, elucidation of the mechanism for Fas-mediated apoptosis has begun (reviewed by Cleveland and Ihle, Cell 81:479-482, 1995; Fraser and Evan, Cell 85:781-784, 1996). FADD, also known as MORT1, is a cytoplasmic protein that has a C-terminal death domain which interacts with Fas and an N-terminal domain that can induce cell death (Chinnaiyan et al., Cell 81:505-512, 1995; Boldin et al., J. Biol. Chem. 270:7795-7798, 1995b). The N-terminus of FADD interacts with MACH/FLICE, an interleukin-1.beta. converting enzyme (ICE) family cysteine protease (caspase) that potently induces apoptosis (Boldin et al., Cell 85:803-815, 1996; Muzio et al., Cell 85:817-827, 1996). Although the details are not yet clear, other caspases, including ICE and CPP32, are sequentially activated to execute the apoptotic dissolution of the cell (Enari et al., Nature 380:723-726, 1996). TNFR1 also interacts with FADD via an adaptor protein termed TRADD (Hsu et al., Cell 84:299-308, 1996). The emerging model from these molecular studies is that Fas, via FADD, directly engages and activates apoptotic ICE family proteases. However, this model fails to explain how Bcl-2 and other physiologic signals may modulate Fas-mediated apoptosis (Fraser and Evan, 1996). It remains possible that other signaling molecules in addition to FADD are involved in Fas-mediated apoptosis.
Fas can also activate the Jun N-terminal kinase/stress activated protein kinase (JNK/SAPK) pathway (Latinis and Koretzky, Blood 87:871-875, 1996; Lenczowski et. al., Mol. Cell. Biol. 17:170-181, 1997; Goillot et. al., Proc. Natl. Acad. Sci. USA 94:3302-3307, 1997). Analogous to the MAP kinase cascade, the prototypical JNK/SAPK pathway involves the sequential activation of the proteins MEKK1, SEK1, JNK, and c-Jun. Other targets of the JNK pathway include the transcription factors Elk-1 and ATF-2 (reviewed by Kyriakis and Avruch, Ann. N.Y. Acad. Sci. 766:303-319, 1996). This pathway was initially characterized by the ability of UV irradiation and transforming Ha-Ras to activate the AP-1 transcription factor; subsequently it was shown that TNF-.alpha. and other stress-activated signals may also activate this pathway. The significance of Fas-mediated JNK activation is currently unclear. One hypothesis is that activation of the JNK pathway contributes to Fas-mediated apoptosis (Goillot et. al., 1997). Dominant negative constituents of the JNK pathway can block stress- and TNF-induced apoptosis in several cell lines, suggesting that activation of JNK pathway is required for these apoptotic inducers (Verheij et al., Nature 380:75-79, 1996). Similarly, in PC12 cells that undergo apoptosis in response to nerve growth factor withdrawal, activation of the JNK pathway in concert with the suppression of the ERK pathway is critical to induce programmed cell death (Xia et al., Science 270:1326-1331, 1995). Alternatively, Fas-mediated JNK activation may drive cellular proliferation via activation of the proto-oncogene c-Jun and AP-1 transcriptional activity (Rathmell et al., Cell 87:319-329, 1996).
Recently, Liu et. al. have demonstrated that overexpression of FADD, the established downstream signal transducer of Fas, cannot activate JNK but that two other proteins engaged by TNFR1--RIP and TRAF2--are responsible for JNK activation by TNF (Liu et al., Cell 87:565-576, 1996). This raises the question of whether Fas also engages other proteins to activate the JNK pathway.
There exists a need to influence the Fas-mediated apoptosis and JNK signal transduction pathways to treat disease. There also exists a need to identify the gene(s) responsible for increased or decreased signal transduction and to provide therapies for treating diseases resulting from aberrant signal transduction.